19 files changed, 87 insertions, 22 deletions
diff --git a/absl/base/config.h b/absl/base/config.h
index 58097b0d..94f7fcb5 100644
--- a/absl/base/config.h
+++ b/absl/base/config.h
@@ -56,6 +56,25 @@
 #include <cstddef>
 #endif  // __cplusplus
 
+// ABSL_INTERNAL_CPLUSPLUS_LANG
+//
+// MSVC does not set the value of __cplusplus correctly, but instead uses
+// _MSVC_LANG as a stand-in.
+// https://docs.microsoft.com/en-us/cpp/preprocessor/predefined-macros
+//
+// However, there are reports that MSVC even sets _MSVC_LANG incorrectly at
+// times, for example:
+// https://github.com/microsoft/vscode-cpptools/issues/1770
+// https://reviews.llvm.org/D70996
+//
+// For this reason, this symbol is considered INTERNAL and code outside of
+// Abseil must not use it.
+#if defined(_MSVC_LANG)
+#define ABSL_INTERNAL_CPLUSPLUS_LANG _MSVC_LANG
+#elif defined(__cplusplus)
+#define ABSL_INTERNAL_CPLUSPLUS_LANG __cplusplus
+#endif
+
 #if defined(__APPLE__)
 // Included for TARGET_OS_IPHONE, __IPHONE_OS_VERSION_MIN_REQUIRED,
 // __IPHONE_8_0.
@@ -807,6 +826,29 @@ static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
 #define ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION 1
 #endif
 
+// ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
+//
+// Prior to C++17, static constexpr variables defined in classes required a
+// separate definition outside of the class body, for example:
+//
+// class Foo {
+//   static constexpr int kBar = 0;
+// };
+// constexpr int Foo::kBar;
+//
+// In C++17, these variables defined in classes are considered inline variables,
+// and the extra declaration is redundant. Since some compilers warn on the
+// extra declarations, ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL can be used
+// conditionally ignore them:
+//
+// #ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
+// constexpr int Foo::kBar;
+// #endif
+#if defined(ABSL_INTERNAL_CPLUSPLUS_LANG) && \
+    ABSL_INTERNAL_CPLUSPLUS_LANG < 201703L
+#define ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL 1
+#endif
+
 // `ABSL_INTERNAL_HAS_RTTI` determines whether abseil is being compiled with
 // RTTI support.
 #ifdef ABSL_INTERNAL_HAS_RTTI
@@ -868,5 +910,4 @@ static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
 #define ABSL_INTERNAL_HAVE_ARM_NEON 1
 #endif
 
-
 #endif  // ABSL_BASE_CONFIG_H_
diff --git a/absl/base/exception_safety_testing_test.cc b/absl/base/exception_safety_testing_test.cc
index a59be29e..a87fd6a9 100644
--- a/absl/base/exception_safety_testing_test.cc
+++ b/absl/base/exception_safety_testing_test.cc
@@ -701,7 +701,10 @@ struct BasicGuaranteeWithExtraContracts : public NonNegative {
 
   static constexpr int kExceptionSentinel = 9999;
 };
+
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
 constexpr int BasicGuaranteeWithExtraContracts::kExceptionSentinel;
+#endif
 
 TEST(ExceptionCheckTest, BasicGuaranteeWithExtraContracts) {
   auto tester_with_val =
diff --git a/absl/base/internal/cycleclock.cc b/absl/base/internal/cycleclock.cc
index f6e64242..902e3f5e 100644
--- a/absl/base/internal/cycleclock.cc
+++ b/absl/base/internal/cycleclock.cc
@@ -26,6 +26,7 @@
 #include <chrono>  // NOLINT(build/c++11)
 
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/internal/unscaledcycleclock.h"
 
 namespace absl {
@@ -34,8 +35,10 @@ namespace base_internal {
 
 #if ABSL_USE_UNSCALED_CYCLECLOCK
 
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
 constexpr int32_t CycleClock::kShift;
 constexpr double CycleClock::kFrequencyScale;
+#endif
 
 ABSL_CONST_INIT std::atomic<CycleClockSourceFunc>
     CycleClock::cycle_clock_source_{nullptr};
diff --git a/absl/base/internal/fast_type_id.h b/absl/base/internal/fast_type_id.h
index 3db59e83..a547b3a8 100644
--- a/absl/base/internal/fast_type_id.h
+++ b/absl/base/internal/fast_type_id.h
@@ -28,8 +28,10 @@ struct FastTypeTag {
   constexpr static char dummy_var = 0;
 };
 
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
 template <typename Type>
 constexpr char FastTypeTag<Type>::dummy_var;
+#endif
 
 // FastTypeId<Type>() evaluates at compile/link-time to a unique pointer for the
 // passed-in type. These are meant to be good match for keys into maps or
diff --git a/absl/base/internal/spinlock.cc b/absl/base/internal/spinlock.cc
index 35c0696a..9b5ed6e4 100644
--- a/absl/base/internal/spinlock.cc
+++ b/absl/base/internal/spinlock.cc
@@ -19,6 +19,7 @@
 #include <limits>
 
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/internal/atomic_hook.h"
 #include "absl/base/internal/cycleclock.h"
 #include "absl/base/internal/spinlock_wait.h"
@@ -66,12 +67,14 @@ void RegisterSpinLockProfiler(void (*fn)(const void *contendedlock,
   submit_profile_data.Store(fn);
 }
 
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
 // Static member variable definitions.
 constexpr uint32_t SpinLock::kSpinLockHeld;
 constexpr uint32_t SpinLock::kSpinLockCooperative;
 constexpr uint32_t SpinLock::kSpinLockDisabledScheduling;
 constexpr uint32_t SpinLock::kSpinLockSleeper;
 constexpr uint32_t SpinLock::kWaitTimeMask;
+#endif
 
 // Uncommon constructors.
 SpinLock::SpinLock(base_internal::SchedulingMode mode)
diff --git a/absl/container/fixed_array.h b/absl/container/fixed_array.h
index 839ba0bc..2aefae3b 100644
--- a/absl/container/fixed_array.h
+++ b/absl/container/fixed_array.h
@@ -489,12 +489,14 @@ class FixedArray {
   Storage storage_;
 };
 
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
 template <typename T, size_t N, typename A>
 constexpr size_t FixedArray<T, N, A>::kInlineBytesDefault;
 
 template <typename T, size_t N, typename A>
 constexpr typename FixedArray<T, N, A>::size_type
     FixedArray<T, N, A>::inline_elements;
+#endif
 
 template <typename T, size_t N, typename A>
 void FixedArray<T, N, A>::NonEmptyInlinedStorage::AnnotateConstruct(
diff --git a/absl/container/internal/hashtablez_sampler.cc b/absl/container/internal/hashtablez_sampler.cc
index c6036d51..efc1be58 100644
--- a/absl/container/internal/hashtablez_sampler.cc
+++ b/absl/container/internal/hashtablez_sampler.cc
@@ -32,7 +32,10 @@
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace container_internal {
+
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
 constexpr int HashtablezInfo::kMaxStackDepth;
+#endif
 
 namespace {
 ABSL_CONST_INIT std::atomic<bool> g_hashtablez_enabled{
diff --git a/absl/container/internal/raw_hash_set.cc b/absl/container/internal/raw_hash_set.cc
index 61bdb773..c63a2e02 100644
--- a/absl/container/internal/raw_hash_set.cc
+++ b/absl/container/internal/raw_hash_set.cc
@@ -31,7 +31,9 @@ alignas(16) ABSL_CONST_INIT ABSL_DLL const ctrl_t kEmptyGroup[16] = {
     ctrl_t::kEmpty,    ctrl_t::kEmpty, ctrl_t::kEmpty, ctrl_t::kEmpty,
     ctrl_t::kEmpty,    ctrl_t::kEmpty, ctrl_t::kEmpty, ctrl_t::kEmpty};
 
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
 constexpr size_t Group::kWidth;
+#endif
 
 // Returns "random" seed.
 inline size_t RandomSeed() {
diff --git a/absl/numeric/int128.cc b/absl/numeric/int128.cc
index 2f4e081f..8cdcbf05 100644
--- a/absl/numeric/int128.cc
+++ b/absl/numeric/int128.cc
@@ -332,6 +332,7 @@ std::ostream& operator<<(std::ostream& os, int128 v) {
 ABSL_NAMESPACE_END
 }  // namespace absl
 
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
 namespace std {
 constexpr bool numeric_limits<absl::uint128>::is_specialized;
 constexpr bool numeric_limits<absl::uint128>::is_signed;
@@ -381,3 +382,4 @@ constexpr int numeric_limits<absl::int128>::max_exponent10;
 constexpr bool numeric_limits<absl::int128>::traps;
 constexpr bool numeric_limits<absl::int128>::tinyness_before;
 }  // namespace std
+#endif
diff --git a/absl/status/status.cc b/absl/status/status.cc
index c66009d6..89749a52 100644
--- a/absl/status/status.cc
+++ b/absl/status/status.cc
@@ -195,7 +195,9 @@ const std::string* Status::EmptyString() {
   return &empty.str;
 }
 
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
 constexpr const char Status::kMovedFromString[];
+#endif
 
 const std::string* Status::MovedFromString() {
   static std::string* moved_from_string = new std::string(kMovedFromString);
diff --git a/absl/strings/cord.cc b/absl/strings/cord.cc
index 04dc7b90..85a67a08 100644
--- a/absl/strings/cord.cc
+++ b/absl/strings/cord.cc
@@ -160,7 +160,9 @@ static CordRep* CordRepFromString(std::string&& src) {
 // --------------------------------------------------------------------
 // Cord::InlineRep functions
 
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
 constexpr unsigned char Cord::InlineRep::kMaxInline;
+#endif
 
 inline void Cord::InlineRep::set_data(const char* data, size_t n) {
   static_assert(kMaxInline == 15, "set_data is hard-coded for a length of 15");
diff --git a/absl/strings/cord_buffer.cc b/absl/strings/cord_buffer.cc
index fd4045bd..fad6269c 100644
--- a/absl/strings/cord_buffer.cc
+++ b/absl/strings/cord_buffer.cc
@@ -21,8 +21,10 @@
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
 constexpr size_t CordBuffer::kDefaultLimit;
 constexpr size_t CordBuffer::kCustomLimit;
+#endif
 
 ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/cord_rep_btree.cc b/absl/strings/internal/cord_rep_btree.cc
index 2b592b47..cacbf3da 100644
--- a/absl/strings/internal/cord_rep_btree.cc
+++ b/absl/strings/internal/cord_rep_btree.cc
@@ -33,7 +33,9 @@ namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace cord_internal {
 
-constexpr size_t CordRepBtree::kMaxCapacity;  // NOLINT: needed for c++ < c++17
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
+constexpr size_t CordRepBtree::kMaxCapacity;
+#endif
 
 namespace {
 
diff --git a/absl/strings/internal/cord_rep_ring.cc b/absl/strings/internal/cord_rep_ring.cc
index db1f63fa..af2fc768 100644
--- a/absl/strings/internal/cord_rep_ring.cc
+++ b/absl/strings/internal/cord_rep_ring.cc
@@ -129,7 +129,9 @@ class CordRepRing::Filler {
   index_type pos_;
 };
 
-constexpr size_t CordRepRing::kMaxCapacity; // NOLINT: needed for c++11
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
+constexpr size_t CordRepRing::kMaxCapacity;
+#endif
 
 bool CordRepRing::IsValid(std::ostream& output) const {
   if (capacity_ == 0) {
diff --git a/absl/strings/internal/cordz_info.cc b/absl/strings/internal/cordz_info.cc
index c891d0ed..dac3fd8b 100644
--- a/absl/strings/internal/cordz_info.cc
+++ b/absl/strings/internal/cordz_info.cc
@@ -34,7 +34,9 @@ namespace cord_internal {
 
 using ::absl::base_internal::SpinLockHolder;
 
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
 constexpr int CordzInfo::kMaxStackDepth;
+#endif
 
 ABSL_CONST_INIT CordzInfo::List CordzInfo::global_list_{absl::kConstInit};
 
diff --git a/absl/strings/internal/str_format/extension.cc b/absl/strings/internal/str_format/extension.cc
index 484f6ebf..f93153d5 100644
--- a/absl/strings/internal/str_format/extension.cc
+++ b/absl/strings/internal/str_format/extension.cc
@@ -33,6 +33,8 @@ std::string FlagsToString(Flags v) {
   return s;
 }
 
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
+
 #define ABSL_INTERNAL_X_VAL(id) \
   constexpr absl::FormatConversionChar FormatConversionCharInternal::id;
 ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(ABSL_INTERNAL_X_VAL, )
@@ -45,17 +47,14 @@ constexpr absl::FormatConversionChar FormatConversionCharInternal::kNone;
 ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(ABSL_INTERNAL_CHAR_SET_CASE, )
 #undef ABSL_INTERNAL_CHAR_SET_CASE
 
-// NOLINTNEXTLINE(readability-redundant-declaration)
 constexpr FormatConversionCharSet FormatConversionCharSetInternal::kStar;
-// NOLINTNEXTLINE(readability-redundant-declaration)
 constexpr FormatConversionCharSet FormatConversionCharSetInternal::kIntegral;
-// NOLINTNEXTLINE(readability-redundant-declaration)
 constexpr FormatConversionCharSet FormatConversionCharSetInternal::kFloating;
-// NOLINTNEXTLINE(readability-redundant-declaration)
 constexpr FormatConversionCharSet FormatConversionCharSetInternal::kNumeric;
-// NOLINTNEXTLINE(readability-redundant-declaration)
 constexpr FormatConversionCharSet FormatConversionCharSetInternal::kPointer;
 
+#endif  // ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
+
 bool FormatSinkImpl::PutPaddedString(string_view value, int width,
                                      int precision, bool left) {
   size_t space_remaining = 0;
diff --git a/absl/strings/internal/string_constant.h b/absl/strings/internal/string_constant.h
index b358efdd..f68b17d7 100644
--- a/absl/strings/internal/string_constant.h
+++ b/absl/strings/internal/string_constant.h
@@ -50,8 +50,10 @@ struct StringConstant {
                 "The input string_view must point to constant data.");
 };
 
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
 template <typename T>
-constexpr absl::string_view StringConstant<T>::value;  // NOLINT
+constexpr absl::string_view StringConstant<T>::value;
+#endif
 
 // Factory function for `StringConstant` instances.
 // It supports callables that have a constexpr default constructor and a
diff --git a/absl/strings/string_view.cc b/absl/strings/string_view.cc
index d596e08c..adce3be9 100644
--- a/absl/strings/string_view.cc
+++ b/absl/strings/string_view.cc
@@ -207,22 +207,11 @@ string_view::size_type string_view::find_last_not_of(
   return npos;
 }
 
-// MSVC has non-standard behavior that implicitly creates definitions for static
-// const members. These implicit definitions conflict with explicit out-of-class
-// member definitions that are required by the C++ standard, resulting in
-// LNK1169 "multiply defined" errors at link time. __declspec(selectany) asks
-// MSVC to choose only one definition for the symbol it decorates. See details
-// at https://msdn.microsoft.com/en-us/library/34h23df8(v=vs.100).aspx
-#ifdef _MSC_VER
-#define ABSL_STRING_VIEW_SELECTANY __declspec(selectany)
-#else
-#define ABSL_STRING_VIEW_SELECTANY
-#endif
 
-ABSL_STRING_VIEW_SELECTANY
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
 constexpr string_view::size_type string_view::npos;
-ABSL_STRING_VIEW_SELECTANY
 constexpr string_view::size_type string_view::kMaxSize;
+#endif
 
 ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/types/internal/conformance_profile.h b/absl/types/internal/conformance_profile.h
index cf64ff4f..37b017db 100644
--- a/absl/types/internal/conformance_profile.h
+++ b/absl/types/internal/conformance_profile.h
@@ -719,6 +719,7 @@ struct SyntacticConformanceProfileOf {
                                                          type##_support); \
   ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF_IMPL(bool, is_##type)
 
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
 ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF(default_constructible);
 ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF(move_constructible);
 ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF(copy_constructible);
@@ -733,6 +734,7 @@ ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF(greater_equal_comparable);
 ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF(greater_than_comparable);
 ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF(swappable);
 ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF(hashable);
+#endif
 
 #undef ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF
 #undef ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF_IMPL